The invention relates generally to the field of image reconstruction. In particular, the invention relates to techniques for performing image reconstruction using hybrid computed tomography (CT) detectors.
CT imaging systems measure the intensity of X-ray beams passed through a patient from numerous angles. With sufficient angular coverage around the patient, cross-sectional images can be formed revealing the inner structure of the scanned object. The images are typically displayed on a cathode ray tube or a computer screen, and may be printed or reproduced on film. A virtual 3-D image may also be produced by a CT examination.
CT scanners operate by projecting X-ray beams from an X-ray source through an attenuating object, such as a patient. The X-ray beams may be collimated between the source and the object into a fan or cone shape, depending on the configuration of the detector optimal patient exposure, or other factors. The attenuated beams are then detected by a set of detector elements. The detector element produces a signal based on the intensity of the X-ray beams. The measured data are then processed to represent the line integrals of the attenuation coefficients of the object along the ray paths. The processed data are typically called projections. By using reconstruction techniques, such as filtered backprojection, cross-sectional images are formulated from the projections. Adjacent cross-sectional images may be displayed together to render a volume representing the imaged region of the object or patient.
As will be appreciated by those skilled in the art, the attenuation coefficient of a material is a function of two separate events that may occur when an X-ray beam passes through a given length of the material. The first event is known as Compton scatter and denotes the tendency of an X-ray photon passing through the length of the material to be scattered or diverted from the original beam path, with a resultant change in energy. The second event is known as photoelectric absorption and denotes the tendency of an X-ray photon passing through the length of the material to be absorbed by the material.
As one might expect, different materials differ in their scatter and absorption properties, resulting in different attenuation coefficients for different materials. In particular, the probability of Compton scattering depends in part on the electron density of the imaged material and the probability of photoelectric absorption depends in part on the atomic number of the imaged material, i.e., the greater the atomic number, the greater the likelihood of absorption. Furthermore, both the Compton scattering effect and photoelectric absorption depend in part on the energy of the X-ray beam. As a result, materials can be distinguished from one another based upon the relative importance of the photoelectric absorption and Compton scattering effects in X-ray attenuation by the material. In particular, measurement of the attenuation produced by a material at two or more X-ray energy levels or spectra, i.e., multi-energy or multi-spectral CT, may allow for respective Compton scattering and photoelectric absorption contributions to be quantified for a material at the X-ray energy levels employed.
Multi-energy CT scanning refers to the process of acquiring X-ray transmission measurements with two different effective X-ray energies. Often this is achieved by combining measurements at two or more tube voltages (dual kVp). Using two measurements of two different known effective energies it is possible to extract information on tissue and/or material composition. A common strategy is to separate the object into bone equivalent and soft tissue equivalent absorbers. Multi-energy scanning is based upon the principle that in the diagnostic X-ray energy range, essentially all X-ray interactions are either through photoelectric absorption or Compton scattering, which have different energy dependence. These in turn have different dependence on atomic number and electron density. As mentioned above, the probability of Compton scattering is dependent on the X-ray energy and the electron density, while the probability of photoelectric absorption increases rapidly with atomic number and decreases rapidly with increasing photon energy.
Energy discriminating (ED) detectors are generally used in multi-energy CT scanning systems to provide information regarding the energy-distribution of the detected photons, by producing two or more signals corresponding to two or more energy intervals, such as, for example a high energy signal and a low energy signal. As will be appreciated by those skilled in the art, ED detectors provide spatial information in conjunction with information regarding the physical density and/or effective atomic number of the material or materials within the imaging volume. Using the spatial and density and/or atomic number information, an operator may reconstruct images that predominantly display selected materials, such as bone, soft tissue, or contrast agent, which differ in their atomic number or density. In this manner, a bone image, a soft tissue image, a contrast agent image, and so forth may be reconstructed which predominantly displays the material of interest. These images may in turn be associated to form a volume rendering of the material of interest which may be useful in determining bone density or deterioration, soft tissue damage, contrast agent perfusion, and so forth. ED detectors may be used with a single source energy or with multiple source energies similar to a dual kVp CT system.
On the other hand, conventional CT detectors are referred to as Energy Integrating (EI) detectors. EI detectors produce an electronic signal that is proportional to the total amount of absorbed X-ray energy in each view. Consequently, the detector signal does not contain any information regarding the energy distribution of the individual photons.
A number of reconstruction techniques have been proposed that either use energy integrating (EI) detectors or energy discriminating (ED) detectors to reconstruct image data. Reconstruction using ED detectors comprises performing material decomposition on the projection measurements, during image reconstruction, or after image reconstruction. With the pre-reconstruction decomposition, material specific (e.g. bone and soft tissue, water and bone, water and barium or Compton scatter and Photo electric) projections are computed at each view angle and from each set, material specific images are reconstructed. An advantage of this approach is that beam hardening artifacts are prevented. With post-reconstruction multi-energy processing, each image may have beam hardening artifacts that are not removed in the material decomposition.
It would be desirable to develop a technique that combines the energy information provided by ED detector cells with the high flux capability and high signal to noise (SNR) ratio provided by EI detector cells for reconstructing image data in a CT system. In addition, it would be desirable to develop techniques for reconstructing image data comprising EI measurement data and ED measurement data using a CT detector comprising a combination of EI detector cells and ED detector cells.